1. Field of the Invention
This invention relates to a semiconductor memory and method of manufacturing a semiconductor memory, and in particular, to a semiconductor memory and method of manufacturing such a memory using a thin film of high-dielectric-constant or ferroelectric material as a capacitor insulator for a storage capacitor.
The term high dielectric constant or ferroelectric material thin film refers to a high-dielectric-constant or ferroelectric thin film forming the capacitor insulator, and capacitors using these insulating thin films are referred to by the general name of high-dielectric-constant or ferroelectric thin film capacitors as described in detail hereafter.
2. Prior Art
Semiconductor memory capacitors using high-dielectric-constant materials as capacitor insulating films are now being considered for use in applications requiring high electrostatic capacitances of small surface area, in particular large-scale DRAM, due to the fact that they have a higher electrostatic capacitance per unit surface area than a capacitor using a conventional insulating film such as a silicon oxide film or silicon nitride film.
For example, according to xe2x80x9cIEEE International Electron Device Meeting pp.823-826 (1991)xe2x80x9d, there have been reports of using (Ba, Sr)TiO3 (abbreviated hereafter as BST) as a high ferroelectric material.
Capacitors of identical structure using ferroelectric materials as capacitor insulating films are also being considered for use as large-scale, non-volatile memories. For example, according to xe2x80x9c1995 Symposium on VLSI Technology Digest of Technical Papers, pp.123-124xe2x80x9d, has mentioned the use of Pb(Zr, Ti)O3 (referred to hereafter as PZT) as a ferroelectric material. The electrode material in these cases was a noble metal, Pt, as in the case of BST.
When semiconductor memories using these materials are manufactured, after manufacturing the capacitor, an interconnecting layer which makes electrical connection to this capacitor, and an interconnecting layer relating to peripheral circuit parts which perform electrical conversions between the memory cells and the outside of the memory chip, are formed. An insulating film must be formed to provide electrical insulation between these interconnecting layers and between these interconnecting layers and the capacitor, and this process must be performed in a reducing or slightly oxidizing atmosphere to avoid deterioration of the interconnecting layers. However, the capacitor is known to suffer considerable damage as a result of this process.
For example, according to xe2x80x9cMaterial Research Society Symposium Proceedings, Vol. 310, pp.151-156 (1993)xe2x80x9d, it is reported that PZT which is a ferroelectric material loses its ferroelectricity and leakage current increases when an SiO2 film is formed by CVD as an inter-layer insulating film.
Further, in the memory manufacturing process, annealing in a hydrogen atmosphere is carried out as a final step to ensure reliability of the metal interconnection layer and the transistor formed in the layer below the capacitor. This hydrogen treatment is known to have an effect on capacitor characteristics as in the case of the inter-layer insulating film forming step.
For example, according to xe2x80x9c8th International Symposium on Integrated Ferroelectrics, 11c (1996)xe2x80x9d, when SrBi2Ta2O9 (referred to hereafter as SBT) is used as a ferroelectric material, there is considerable deterioration of ferroelectricity when treatment in a hydrogen atmosphere is performed, and attempts have been made to repair the damage due to this hydrogen treatment by annealing in an oxygen atmosphere.
However, it was found that there are problems in applying the above methods to an actual manufacturing process, i.e.:
(1) The materials used for the metal interconnection formed after the capacitor manufacturing step, such as aluminum, etc., are not resistant to the high temperature oxidizing atmosphere.
(2) In the oxygen annealing of the following step, the oxygen concentration decreases due to the thickness of the inter-layer insulating film, and recovery from damage is also less. According to experiments carried out by the inventor, if hydrogen treatment has been carried out to ensure reliability of the transistor, capacitor characteristics do not recover sufficiently once they have deteriorated even if annealing is subsequently performed in oxygen, and repair is difficult.
One type of deterioration is capacitor delamination (delamination of the upper electrode from the capacitor insulating layer). Due to this phenomenon, in the manufacture of large-scale memories which employ a considerable amount of fine interconnections, all products in the same manufacturing lot become defective, yield declines and reliability drastically decreases.
In addition to the above problems, it was found that compared to capacitors which have not been treated, capacitors treated in a hydrogen atmosphere suffer considerable degrading of electrical characteristics due to voltage stress and alternating electric field stress, and there are serious problems with regard to their reliability.
Japanese Published Unexamined Application No. Hei 8-55967 (published on Feb. 27, 1996) which relates to a method of manufacturing a ferroelectric thin film capacitor, discloses how leakage current is reduced by annealing oxygen vacancies in the ferroelectric thin film (in an oxidizing atmosphere). However, no mention is made of the problem of electrode delamination.
It is therefore an object of this invention to resolve the above problems, and in particular, to provide a semiconductor memory comprising a capacitor having low leakage current, high capacitance and high remnant polarization, and a method of manufacturing such a memory.
To achieve the above objects, as a result of various experiments carried out to study degrading of capacitor characteristics, the inventor found the main cause to be a structural transformation in the vicinity of the boundary between the upper electrode and the high-dielectric-constant or ferroelectric material functioning as the capacitor insulating film, in particular the production of oxygen vacancies on the surface of the metal oxide which is the high-dielectric-constant or ferroelectric material. It was found that a large number of these oxygen vacancies were present at the boundary between the upper electrode (formed after the high-dielectric-constant or ferroelectric thin film) and the high-dielectric-constant or ferroelectric thin film. It was also found that deterioration of adhesive properties between the noble metal of the upper electrode and the high-dielectric-constant or ferroelectric material (delamination of upper electrode) was strongly correlated with this oxygen vacancy concentration.
The inventor attempted to actively oxidize the high-dielectric-constant or ferroelectric material-electrode boundary after forming the capacitor electrodes (particularly the upper electrode). As a result, degradation and delamination were suppressed in the step for forming the insulating film of the high-dielectric-constant or ferroelectric material capacitor, which is the subsequent step, and long-term reliability was improved. In other words, it was confirmed that carrying out oxygen treatment before hydrogen treatment after forming the capacitor was extremely effective compared to the prior art method of performing hydrogen treatment after forming the capacitor.
This invention, which was conceived on the basis of the above observations, therefore provides a semiconductor memory comprising an integrated capacitor having a structure comprising an upper electrode, lower electrode and high ferroelectric oxide thin film which functions as a capacitor insulating film held between these electrodes, and a protective insulating film formed by plasma treatment covering this capacitor structure, wherein a layer with additional oxygen is formed on the surface of the high ferroelectric oxide thin film functioning as the capacitor insulating film.